The catalytic oxidation of alcohols selectively to carbonyl compounds is essential for many important transformations in the synthetic organic chemistry. A large number of oxidants have been reported in the literature and most of them are based on transition metal oxides such as chromium and manganese (S. Kirk-Othmer Mitchell, Enciclopedia of Chemical Technology, 4th ed., Wiley-Interscience, New York, Vol. 2, p 481, (1992); Hudlicky, M. “Oxidations in Organic Chemistry”, ACS Monograph No. 186 American Chemical Society Washington D.C. (1990); Sheldon R. A., Kochi J. K. Metal Catalized Oxidation of Organic Compounds. New York, Academic Press, 1981; Ley, S. V., Madin, A. In comprehensive Organic Synthesis, Trost B., Fleming, I., Eds.; Pergamon Oxford, 1991; Vol 7, p 251; Mijs, W. J., DeJonge, C. R. H. I. Organic Synthesis by Oxidation with Metal Compounds; Plenum: New York, 1968). The methods described require the use of stoichiometric amounts of inorganic oxidants, generally highly toxic chromium or manganese compounds, which creates serious problems related to their handling and disposal.
A particularly convenient procedure for the oxidation of primary and secondary alcohols is reported by Anelli and co-workers (J. Organic Chemistry, 1987, 52, 2559; J. Organic Chemistry, 1989, 54, 2970). The oxidation has been carried out in a two-phase system (CH2Cl2-water) utilizing the TEMPO as a catalyst and cheap and readily accessible NaOCl as an oxidant. The co-catalyst KLBr enhances the reaction rate and the aqueous phase is buffered at pH 8.5-9.5 using NaHCO3. The use of a quatemary ammonium salt as a phase transfer catalyst furthers the oxidation of alcohols to carboxylic acids. The same procedure was modified by using NaClO2 as the oxidant in the presence of catalytic amounts of TEMPO and NaOCl. This led to the formation of the carboxylic acid as the main product (U.S. Pat. No. 6,127,573).
Prakash et al. in U.S. Pat. No. 5,856,584 report a similar procedure for oxidation of 3,3-Dimethyl-1-butanol. According to this procedure the 3,3-Dimethyl-1-butanol is oxidized to 3,3-dimethyl-1-butanal with NaOCl in a two-phase system using CH2Cl2 as a reaction solvent. The stable 2,2,6,6-tetramethyl-1-piperidinyloxy free radical and a KBr are used as an efficient catalytic system to produce the desired aldehyde in 80% isolated yield.
A particularly efficient method for oxidation of primary and secondary alcohols was also reported in U.S. application Ser. No. 60/443,749. The oxidation is carried out at temperature −5-0° C. using the TEMPO as a catalyst, Na2B4O7 as a co-catalyst and NaOCl as an oxidant. The procedure does not require reaction solvent and is carried out without KBr promoter. The aqueous phase is buffered at pH 8.7-9.2 using NaHCO3. This procedure gives small amount of chlorinated hydrocarbon impurities.
The search for efficient, easily accessible catalysts and “clean” oxidants such as hydrogen peroxide, hydroperoxides or molecular oxygen for industrial applications is still a challenge (Dijksman, A., Arends I. W. C. E. and Sheldon R., Chem. Commun., 1999, 1591-1592; Marko I. E., P. R. Giles, Tsukazaki M., Brown S. M. and Urch C. J., Science, 19696, 274, 2044). A large number of transition metal complexes and oxidants have been reported to catalyze the selective oxidation of primary alcohols to aldehydes with varying levels of effectiveness such as RuCl3-NaBrO3 (Konemoto S., Tomoioka S., Oshima K.), Bull. Chem. Soc. Japan. 1986. V. 59. N1, P. 105), Bu4NRuO4-4-Methylmorpholine N-oxide (Griffith W. P., Ley S. V., Whitcombe G. P., White A. D)., Chem. Commun. 1987, N21, p. 1625), H2O2 and tert-Butylhydroperoxide (t-BuOOH) (Y. Tsuji, T. Ohta, T. Ido et al.), J. Organometallic Chemistry, 270, 333 (1984), (T. M. Jiang, J. C. Hwang, H. O. Ho, C. Y. Chen), J. Chin. Chem. Soc., 35, 135, (1988). The methods described have only limited use since the overall yields are low and some of them require the application of precious metal complexes or expensive primary oxidants.
In the area of the aerobic oxidation of alcohols, very few efficient systems are currently available. A catalyst system based on TEMPO and Mn(NO3)2—Co(NO3)2 or Mn(NO3)2—Cu(NO3)2 was recently reported (A. Cecchetto, F. Fontana, F. Minisci and F. Recupero) Tetrahedron Letters, 42, 6651-6653 (2001). The oxidation requires diluted solutions of the starting alcohol in acetic acid solvent (in the range 6-10% v/v) and takes place at ambient temperatures and at atmospheric pressure of oxygen. A serious drawback of the method is the rapid deactivation of the catalyst at higher alcohol concentrations, resulting in the catalyst system being virtually inactive Because of this, direct commercial application is not economically feasible as higher alcohol concentrations are typically required in such processes.
Despite the extensive work reported in the area of the selective oxidation of primary alcohols there is still a continuous need for developing highly efficient and economical oxidation methods using molecular oxygen as environmentally friendly oxidants. It is the object of the present invention to provide such an oxidation method.